A radio frequency (RF) power amplifier is the last amplification stage in a radio transmitter. It is a particularly non-linear amplifier. In other words, a power amplifier's complex gain is not constant for all input power levels.
This non-linearity is problematic. For an RF signal with an AM component, non-linear amplification introduces intermodulation distortion (IMD), which is strictly limited by government regulation. In fact, IMD regulation forces many power amplifiers to be operated at low power, low efficiency levels. Clearly it is undesirable to limit amplifier output power if other methods exist for reducing IMD.
One method is to actively linearize the response of the amplification path as a whole to compensate for non-linearity in the power amplifier. A complex error signal is calculated by comparing the amplification path's output signal to its input signal. A compensating signal is then generated from the error signal. In adaptive feedforward compensation, the compensating signal is added to the amplification path's output signal. In adaptive predistortion, the compensating signal adjusts a non-linear device placed in series ahead of the power amplifier. In vector feedback linearization, the compensating signal adjusts the complex gain of a vector modulator in the amplification path, maintaining a constant amplification path gain even while the gain of the power amplifier varies. The present invention relates to this last method, vector feedback linearization.
Conventional vector feedback linearization systems suffer from a number of disadvantages.
First, they are sensitive to power variations in the input signal. It is difficult to achieve both rapid convergence and feedback stability over the whole range of input power levels.
Second, they are not robust. They will fail to converge from certain gain states, particularly initial gain states whose phase and amplitude errors are large. For this reason, conventional vector feedback systems must include an alignment amplifier that is manually calibrated to prevent the amplification path from entering gain states that lead to instability or divergence. Such alignment devices generally include a series combination of an adjustable attenuator and an adjustable phase shifter.
Third, they are sensitive to mismatch errors. Time delay differences between circuit loops introduce systematic measurement and control errors. Painstaking manual calibration is generally required to eliminate the effects of these time delay differences over the limited bandwidth of interest.
What is needed is a robust vector feedback system that is less sensitive to varying input power levels. It should converge quickly but remain stable regardless of the input power level or gain error. It should self-calibrate to remove the undesirable effects of loop time delay mismatches.